Custom electrodes for molecular memory and logic devices

ABSTRACT

A method is provided for fabricating molecular electronic devices comprising at least a bottom electrode and a molecular switch film on the bottom electrode. The method includes forming the bottom electrode by a process including: cleaning portions of the substrate where the bottom electrode is to be deposited; pre-sputtering the portions; depositing a conductive layer on at least the portions; and cleaning the top surface of the conductive layer. Advantageously, the conductive electrode properties include: low or controlled oxide formation (or possibly passivated), high melting point, high bulk modulus, and low diffusion. Smooth deposited film surfaces are compatible with Langmuir-Blodgett molecular film deposition. Tailored surfaces are further useful for SAM deposition. The metallic nature gives high conductivity connection to molecules. Barrier layers may be added to the device stack, i.e., Al 2 O 3  over the conductive layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to U.S. Pat. No. 6,459,095, issuedOct. 1, 2002, entitled “Chemically Synthesized and Assembled ElectronicDevices”, which is directed to the formation of nanowires used fornanoscale computing and memory circuits. The present application is alsorelated to U.S. Pat. No. 6,314,019, issued Nov. 6, 2001, entitled“Molecular Wire Crossbar Interconnect (MWCI) for Signal Routing andCommunications”, and to U.S. Pat. No. 6,128,214, entitled “MolecularWire Crossbar Memory”, issued on Oct. 3, 2000, as well as toapplications Ser. No. 09/280,045, entitled “Molecular Wire CrossbarLogic (MWCL)”, and Ser. No. 09/280,188, entitled “Molecular WireTransistor (MWT)”, both filed on Mar. 29, 1999, which are all directedto various aspects of memory and logic circuits utilized innanocomputing. The present application is also related to applicationSer. No. 09/823,195, filed Mar. 29, 2001, entitled “Bistable MolecularMechanical Devices with a Band Gap Change Activated by an Electric Fieldfor Electronic Switching, Gating, and Memory Applications”, and to U.S.Pat. No. 6,458,621, entitled “Batch Fabricated Molecular ElectronicDevices with Cost-Effective Lithographic Electrodes”, issued on Oct. 1,2002. The foregoing items are all incorporated herein by reference.

TECHNICAL FIELD

The present application is generally directed to nanoscale computing andmemory circuits, and, more particularly, to the formation of wires andcontacts for device applications, specifically, to the fabrication ofelectrodes employed in such devices. The term “nanoscale” reflects thateither the horizontal or vertical dimensions or the electrical pathwaybetween electrodes is measured in nanometers.

BACKGROUND ART

As feature sizes of integrated-circuit devices continue to decrease, itbecomes increasingly difficult to design well-behaved devices. Thefabrication is also becoming increasingly difficult and expensive. Inaddition, the number of electrons either accessed or utilized within adevice is decreasing, which produces increased statistical fluctuationsin the electrical properties. In the limit, device operation depends ona single electron, and traditional device concepts must change.

Molecular electronics has the potential to augment or even replaceconventional devices with electronic elements, can be altered byexternally applied voltages, and has the potential to scale frommicron-size dimensions to nanometer-scale dimensions with little changein the device concept. The molecular switching elements can be formed bysolution techniques; see, e.g., C. P. Collier et al, “ElectronicallyConfigurable Molecular-Based Logic Gates”, Science, Vol. 285, pp.391-394 (16 Jul. 1999) (“Collier I”) and C. P. Collier et al, “A[2]Catenane-Based Solid State Electronically Reconfigurable Switch”,Science, Vol. 289, pp. 1172-1175 (18 Aug. 2000) (“Collier II”). Theself-assembled switching elements may be integrated on top of asemiconductor integrated circuit so that they can be driven byconventional semiconductor electronics in the underlying substrate. Toaddress the switching elements, interconnections or wires are used.

For nanoscale electronic circuits, it is necessary to invent newmaterials with the functions envisioned for them and new processes tofabricate them. Nanoscale molecules with special functions can be usedas basic elements for nanoscale computing and memory applications.

While self-assembled techniques may be employed and while redoxreaction-based molecules may be used, such as rotaxanes,pseudo-rotaxanes, and catenanes, other techniques for assembling thedevices and other molecular systems may alternatively be employed. Anexample of such other techniques comprises lithographic techniquesadapted to feature sizes in the micrometer-size range, as well asfeature sizes in the nanometer-size range. An example of other molecularsystems involves electric-field-induced band gap changes, such asdisclosed and claimed in patent application Ser. No. 09/823,195, filedMar. 29, 2001, which is incorporated herein by reference. While priorreferences have employed the term “band gap”, this term more preciselyis used for semiconductors. The corresponding term with regard tomolecules is “HOMO-LUMO gap” (highest occupied molecular orbital-lowestunoccupied molecular orbital), and that is the term that will be usedthroughout.

Examples of molecules used in the electric-field-induced HOMO-LUMO gapchange approach include molecules that evidence:

-   -   (1) molecular conformation change or an isomerization;    -   (2) change of extended conjugation via chemical bonding change        to change the HOMO-LUMO gap; or    -   (3) molecular folding or stretching.

Changing of extended conjugation via chemical bonding change to changethe HOMO-LUMO gap may be accomplished in one of the following ways:

-   -   (a) charge separation or recombination accompanied by increasing        or decreasing HOMO-LUMO localization; or    -   (b) change of extended conjugation via charge separation or        recombination and π-bond breaking or formation.

Molecular electronic devices hold promise for future electronic andcomputational devices. Examples of such molecular electronic devicesinclude, but are not limited to, crossed wires, nanoporous surfaces, andtip addressable circuitry which forms switches, diodes, resistors,transducers, transistors, and other active components. For instance, acrossed wire switch may comprise two wires, or two electrodes, forexample, with a molecular switching species between the two electrodes.Thin single or multiple molecular layers can be formed, for example, byLangmuir-Blodgett (LB) techniques or self-assembled monolayer (SAM) on aspecific site. Well-controlled properties, such as roughness andhydrophilicity of the underlying surface are needed to allow optimal LBfilm formation.

Prior work in the field of molecular electronics has utilized electrodesof gold (Reed et al, Science, Vol. 278, pp. 252-254 (1997); Chen et al,Science, Vol. 286, pp. 1550-1551 (1999)), aluminum (Collier I, supra),and polysilicon (Collier II, supra).

Gold has a low melting point, low bulk modulus, and high diffusivity,making it less stable with respect to external stress and incompatiblewith a standard CMOS process, although it has the advantages of no oxideand the chemical stability of a noble metal. Aluminum forms a poorlycontrolled native oxide that acts as a natural barrier to electronictransport. Polysilicon is a semiconductor with associated semiconductorproperties, giving it lower conductivity than a metal and an oxidebarrier to transport. Polysilicon electrode molecular devices have beenfabricated and shown to display switching (Collier et al, supra).

Platinum is difficult to maintain in a stable form. During the intervalfollowing Pt deposition and preceding the next processing step, an“environmental” film (carbon, etc.) will form on the surface. This is aparticular issue when the active molecular layer may be on the order of20 Å thick, which, for reference, is the same magnitude as a nativesilicon oxide. Working with a just-deposited-film (perhaps the“cleanest” way) is difficult and impractical. Even a “just-deposited”blanket film will require time to move to the next process, which willnot be in ultrahigh vacuum (UHV). Until alternate means of formingpatterned contacts are readily realizable, lithography is presently themost likely technology to use. Shadow masks avoid lithographic process,but are dimensionally limited (to large micron-sized dimensions,sparsely placed). Even nano-imprinting exposes surfaces to organicchemicals that are potentially incompatible with the use of organicactive layers. Therefore, the most practical way to fabricate electrodesincorporating molecules is to pattern the electrode with a flexiblegeometry in a cost-efficient, time efficient, flexible geometry way andthen clean the organics from the surface before subsequent processing.

Thus, a method for preparing platinum, and other conductive electrodes,that avoid most, if not all, of the foregoing problems is required foruse with molecular films for forming molecular electronic devices. Inaddition, it would be an advantage to tailor the surface to desireddevice specifications for use even if lithographic steps are notemployed.

DISCLOSURE OF INVENTION

In accordance with the embodiments disclosed herein, a method isprovided for tailoring the surface of a conductive layer to provide asmooth surface that can be as smooth as the surface of the underlyingsubstrate supporting the conductive layer. By “conductive layer” ismeant a layer comprising a material having a resistivity of less than1375 micro-ohm-cm, wherein the material is capable of forming asolid-state oxide that is stable under ambient conditions. The methodincludes

-   -   (a) depositing the conductive layer on the substrate; and    -   (b) tailoring at least portions of the top surface of the        conductive layer in a plasma to at least smooth the top surface        of the conductive layer, whereby the surface roughness is        essentially the same as that of the substrate.

The terms “tailored” or “tailoring” refer to a process involving thepreparation of the surface preference, and further includes any of thefollowing: (a) actively smoothing, (b) actively oxidizing, whichproduces a very hydrophilic surface good for Langmuir-Blodgett films,(c) actively removing the oxide without re-roughening, and (d) activelypassivating. By “actively” is meant that an operation is performed or asequence of predetermined steps is set in motion to accomplish aspecific desired result.

In accordance with another embodiment, a method of fabricating amolecular electronic device comprising at least a bottom electrode and amolecular switch film thereon is provided. The method comprises:

-   -   (a) providing a substrate;    -   (b) forming the bottom electrode on the substrate, the bottom        electrode comprising a tailored conductive material; and    -   (c) forming the molecular film on at least the bottom electrode,

wherein the bottom electrode is formed by a process including:

-   -   (b1) cleaning portions of the substrate where the bottom        electrode is to be deposited;    -   (b2) pre-sputtering the portions; and    -   (b3) depositing the conductive layer on at least the portions.

In yet another embodiment, after the conductive layer is deposited, thenthe properties of the top surface of the conductive layer are tailored.

In a still further embodiment, a conductive layer having a smoothsurface is provided, wherein the conductive layer essentially replicatesthe smooth surface of the underlying substrate.

In some embodiments, a contact or top electrode is formed over thebottom electrode, which may be oriented at a non-zero angle with respectthereto, such as with a crossbar device, e.g., a switch. For pores,dots, tip addressing, etc., there may be an electrode or alternativelybrief contact may be made, such as with a dot.

Following the last step (depositing the conductive layer or thetailoring step), the molecule or molecular film is formed on thesurface.

In accordance with a further embodiment, a method is provided forforming a conductive layer on a substrate having a first surfaceroughness, with the conductive layer having a second surface roughness,where the second roughness is approximately the same as the firstsurface roughness. The method comprises the steps (b1) to (b3)enumerated above, optionally with the tailoring step.

Advantageously, conductive electrode properties include: a controlledoxide formation (under certain circumstances), a high melting point,high bulk modulus, low diffusion, some degree of stability (whichdepends on surface preparation). Smooth deposited film surfaces arecompatible with Langmuir-Blodgett molecular film deposition. Themetallic nature gives high conductivity connection to molecules. Barrierlayers may be added to the device stack, i.e., Al₂O₃ over the conductivelayer.

The embodiments disclosed and claimed herein, while including thedeposition of the conductive layer, are not to be construed as limitingto just the deposition, but optionally includes the tailoring of theconductive surface through plasma exposure. Such tailoring of theconductive surface is apparently unknown heretofore. Essentially, thephysical structure is combined with chemical features to produce filmsuniquely suited for the application of molecular films through a widevariety of formats, including, but not limited to, Langmuir-Blodgett(LB), self-assembled monolayer (SAM), spin-coat, etc.

The surface may be further tailored to include oxide or no oxide whilemaintaining the low surface roughness, which also changes the weftingproperties, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 d are top plan views of one embodiment of a process forfabricating molecular devices (the embodiment depicted is of a crossedwire device, but the embodiments herein are not so limited);

FIG. 2 is a cross-sectional view (side elevation) taken through thelines 2-2 of FIG. 1 d; and

FIG. 3 is a flow chart depicting the process.

BEST MODES FOR CARRYING OUT THE INVENTION

Definitions

As used herein, the term “self-aligned” as applied to “junction” meansthat the junction that forms the switch and/or other electricalconnection between two electrodes is created wherever portions of thetwo electrodes, either of which may be coated or functionalized,overlap.

The term “device” means a switch, diode, resistor, transducer,transistor, or other electrical element formed with two or moreelectrodes.

The term “self-assembled” as used herein refers to a system thatnaturally adopts some regular pattern because of the identity of thecomponents of the system; the system achieves at least a local minimumin its energy by adopting this configuration.

The term “singly configurable” means that a device can change its stateonly once via an irreversible process such as an oxidation or reductionreaction, such a device can be the basis of a programmable read-onlymemory (PROM), for example.

The term “reconfigurable” means that a device can change its statemultiple times via a reversible process such as an oxidation orreduction;

in other words, the device can be opened and closed multiple times, suchas the memory bits in a random access memory (RAM).

The term “bi-stable” as applied to a molecule means a molecule havingtwo relatively low energy states. The molecule may be eitherirreversibly switched from one state to the other (singly configurable)or reversibly switched from one state to the other (reconfigurable).

“Micron-scale dimensions” refers to dimensions that range from 1micrometer to a few micrometers in size.

“Sub-micron scale dimensions” refers to dimensions that range from 1micrometer down to 0.05 micrometers.

“Nanometer scale dimensions” refers to dimensions that range from 0.1nanometers to 50 nanometers (0.05 micrometers).

“Micron-scale wires” refers to rod or ribbon-shaped conductors orsemiconductors with widths or diameters having the dimensions of 1 to 10micrometers or larger, heights that can range from a few tens ofnanometers to a few micrometers, and lengths of up to severalmicrometers or more.

“Nanometer-scale wires” refers to rod or ribbon-shaped conductors orsemiconductors with widths or diameters having the dimension of 1 to 50nanometers, heights that can range from 0.3 to 100 nm, and lengths of upto several micrometers or more.

Molecular Devices

FIGS. 1 a-1 d depict one embodiment for the fabrication of moleculardevices 10. As shown in FIG. 1 a, a substrate 12 is provided. Next, abottom electrode 14 is formed on a portion of the top surface of thesubstrate 12, as shown in FIG. 1 b. A molecular switch film 16 is formedon the surface of the substrate 12, covering the bottom electrode 14.Finally, a top electrode 18, generally at right angle to the bottomelectrode 14, is applied on the molecular film 16. The completedmolecular device 10 is shown in FIG. 2.

Further details of the formation of a molecular device 10, such as shownin FIG. 2, are available in above-mentioned U.S. Pat. No. 6,458,621.Briefly, the substrate 12 comprises a material selected from the groupconsisting of semiconductors, insulating plastics, polymers, crystallineceramics, and amorphous ceramics. Preferably, the substrate 12 includesa coating 12 a formed thereon, such as an insulating layer formed on asemiconductor wafer, such as SiO₂ on Si.

The bottom electrode 14 comprises a material selected from the groupconsisting of platinum, tungsten, aluminum, polycrystalline silicon,single crystal silicon, amorphous silicon, and conductive polymers.

The molecular film 16 typically comprises a material capable ofswitching/changing in the presence of an applied electric field. Oneexample includes molecular materials based on oxidation/reductionmechanisms, such as rotaxanes, pseudo-rotaxanes, and catenanes.

Another example of the molecule film 16 includes molecular materialsthat evidence an electric field induced HOMO-LUMO (highest occupiedmolecular orbital-lowest unoccupied molecular orbital) gap change andare selected from the group consisting of: (1) molecular conformationchange or an isomerization; (2) change of extended conjugation viachemical bonding change to change the HOMO-LUMO gap; and (3) molecularfolding or stretching, wherein the change of extended conjugation viachemical bonding change to change the HOMO-LUMO gap is selected from thegroup consisting of: (2a) charge separation or recombination accompaniedby increasing or decreasing electron localization; and (2b) change ofextended conjugation via charge separation or recombination and π-bondbreaking or formation.

As noted above, such switch films 16, which are primarily discussed interms of switches, may also be used in a variety of devices, including,but not limited to, diodes, resistors, transducers, transistors, etc.

The top electrode 18 is selected from the same list of materials as thebottom electrode 14, and may be the same or different, with the caveatthat there is usually, but not always, a sticking layer (e.g., Ti). Sucha sticking layer may account for some of the switching activity, i.e.,it may be the difference between the Pt and Ti that is involved in theswitching and so the choice of electrode may well tailor the effect.Also, the top electrode may not even be part of the stack, but ratherpart of a moveable-tip addressable scheme.

Specific examples of top contacts 18 further include circular electrodesand nanopores over the molecular film 16 covered with an electrode. Thenanopore serves to limit the extent of the top contact.

Present Embodiments

The embodiments herein are directed to the improved fabrication ofconductive electrodes, e.g., platinum (Pt), electrodes for use inmolecular electronic devices 10, particularly bottom electrodes 14. Thismaterial has been fabricated as the lower electrode 14 in a device stack10 as shown in FIG. 2. The platinum electrodes 14 have been tested witha 2-station [2] rotaxane molecular film and eicosanoic acid film 16.These molecular devices 10 have displayed both diode behavior and switchbehavior. However, while the following description is specificallydirected to platinum electrodes, the electrode may comprise anyconductive material that forms a solid oxide film that is stable underambient conditions (e.g., standard temperature and pressure—STP).Advantageously, the conductive electrode properties include: low orcontrolled oxide formation (or possibly passivated), high melting point,high bulk modulus, and low diffusion. Further, the conductive materialforming the bottom electrode 14 has a resistivity less than 1375micro-ohm-cm, and may comprise any of the elements in rows 1B-7B and 8of the Periodic Table. Examples include platinium, tungsten, silver,aluminum, copper, nickel, chromium, molybdenum, titanium, and tantalum.Of these, platinum is preferred because it is compatible with CMOS-typeback-end processing and packaging, i.e., oxide/nitride films and hightemperature steps.

The deposition of platinum lower electrodes 14 employing prior artprocedures results in metal layers having a smoothness of 8 to 10 Å (thesmoothness of the coating 12 a is typically about 4 Å). It is noted thatprior deposition techniques that use a typical sticking layer increasethe roughness. Unless the adhesion is carefully controlled, Pt depositedin any useful thickness simply lifts from the surface, especially underliquid conditions such as SAM or LB deposition. Also prior depositionsmake no mention of tailoring the surface; the Pt is just deposited.Herein, the surface is tailored for smoothness, hydrophilicity andbarrier layer.

The following description of the formation of the bottom electrode 14 ona coated substrate 12, 12 a is intended to be exemplary only. FIG. 3illustrates the flow chart for the process disclosed herein.

The substrate 12 is provided (step 30). In the prior art approach, thebottom electrode 14 is formed on the substrate (step 32). Next, themolecular film 16 is formed on the bottom electrode (step 34). Inaccordance with the embodiments disclosed herein, a pattern (if any) isformed for deposition (step 36 a), exposed portions of the substrate 12are cleaned, if necessary (step 36 b), those portions are pre-sputtered(step 36 c), the Pt bottom electrode 14 is deposited on those portions(step 36 d), the pattern is finished, if necessary (step 36 e), residualmaterial, if any, is removed (step 36 f), and the properties of the topsurface of the Pt electrode 14 are cleaned/tailored (step 36 g).Following tailoring of the top surface properties, the molecular film 16is deposited on the Pt electrode 14. The details of the process are nowdescribed.

The substrate 12 comprises <100> SEMI-grade prime silicon wafer(alternatively, an extra smooth substrate, such as cleaved mica, may beused). If a silicon wafer is used, it is cleaned as is conventional inthe semiconductor art for a pre-diffusion clean such as an RCA-clean.

Next, a layer of tight knit, or dense, thermal oxide 12 a is grown onthe silicon wafer 12 (or deposited on a non-silicon wafer). Ifnon-thermal oxide is deposited, it will most likely requiredensification. If a non-silicon substrate, such as mica, is used, thenthe oxide may not be needed, as the substrate may not be electricallyconducting. As is well known, tight knit thermal oxide is grown to beclose-packed, thereby avoiding a separate densification step that wouldincrease the process time.

An oxide, or other suitable material as is known in the art, is neededon silicon to provide an insulating substrate 12 a, and therebyelectrically isolate the subsequent platinum layer from silicon 12.Otherwise, a metal on semiconductor would result, and device propertieswould be more coupled to the substrate, which is less desirable thanmetal on insulator. Direct contact may also produce metal-siliconintermixing. If an insulating non-silicon crystal 12, such as mica, isused, then the insulating layer 12 a is superfluous and can beeliminated, as noted above.

The thermal oxide 12 a is grown to a preferable thickness of about 2,000Å. The layer could be thicker than 2,000 Å, but must not be so thickthat undue stress on the wafer 12 or in the film develops. On the otherhand, the thickness of the thermal oxide 12 a should be greater than1,000 Å for electrical isolation.

A silicon nitride, Si_(x)N_(y), where x=1-3 and y=1-4 (stoichiometricSi_(x)N_(y) is Si₃N₄), could be grown in place of silica, but is lesspreferable, due to the lack of stoichiometric control that is obtainablewith SiO₂.

If desired, a resist is formed and patterned for conventional lift-off(step 36 a). Any of the resist materials commonly employed in this artmay be used. The pattern is the array of one or more bottom electrodes14. The resist is removed from those areas where the platinum is to bedeposited to form the bottom electrodes. Removal of the resist is alsoconventional. A dry etch of the metal would produced a somewhat sharperprofile, which is not necessarily desirable where molecular coverage onthe order of 30 Å is attempted. Indeed, etching (wet/dry/milling, etc.)techniques may be done, although they may involve multiple steps forfabricating desired profiles. Another method of producing a pattern tobe filled with platinum would be the well known shadow-masking process.

Once the areas for Pt deposition have been exposed, these open areas arecleaned (descummed), such as with an oxygen plasma (step 36 b). Thespecific parameters for de-scumming depend on the particular plasmasystem used; for an RIE System 1700, the conditions were 100 mTorr, 100Watts, for 2 minutes, using forward power control. The time may rangefrom 1 to 5 minutes, but no further significant improvement is seenafter 5 minutes. More sputtering, which is undesirable, results fromhigher power. Pressures in the range of 50 to 200 mTorr and powers up to100 Watts have been used.

Next, a pre-sputter of the exposed areas is performed (step 36 c). A 5min. argon (Ar) pre-sputter was performed in an SFI DC Magnetronsputter-system at 6.5 sccm Ar, 0.9 mTorr. This pre-sputter furthercleans the surface (the above O₂ plasma removes organics) and removesenvironmental contaminants. Without this pre-sputter step, thesubsequent Pt layer 14 lifts off under duress, while too much sputteringincreases the surface roughness of the substrate coating 12 a.

The advantage of the pre-sputter step is that no “sticking” layer, oradhesive layer, is required, as is conventional practice in the art, inorder to deposit the platinum layer 14 and maintain it on the surface ofthe substrate 12 or coating 12 a. This avoids the extra steps requiredand potential increased surface roughness resulting from the depositionof these layers(s) otherwise required, e.g., Ti, Cr, Ta, conventionallyused to adhere a platinum layer to a surface.

However, experiments were performed to provide adequate sticking withoutsacrificing smoothness. Further, for films immersed in liquid, it is notalways apparent that the layer is going to peel when dry. For LB coatingand SAM deposition, the Pt film must be well adhered. Some deposited Ptfilms, which seem to be adequately adhered without the process disclosedherein, simply roll up like a window shade when the substrate isimmersed in fluid.

In a preferred embodiment, the platinum layer 14 is blanket-depositedeverywhere, using, for example, a DC magnetron sputtering system (step36 d). As an example of operating parameters, present sample values forcleaned and reconfigured system are: cathode: 6.7 A, 6.7 V; beam: 15 mA,348 V; accelerator 1.3 mA, 150.5 V; neutralizer: 5.61 A; emission: 16.8mA to deposit a layer of Pt about 1,000 Å thick. The Pt layer 14 can bethinner or thicker than 1,000 Å, but must be thick enough to providegood conduction, but not so thick as to provide a large step for themolecular switch film 16 to cover. By “good” conduction is meant thatthe platinum layer 14 can pass a desired current through a probe. Thethickness of the Pt layer 14 is in the range of 50 to 5,000 Å, Nolumps/asperities of platinum were observed on the surface from thissystem for a thickness of 1,000 Å. A desired profile without sharp edgesis achieved through lift-off techniques. Fine line liftoff is achievedwith thinner depositions, without undue experimentation. While liftoffis preferred, shadow-masking and etching may alternatively be performed.

In the preferred embodiment, the formation of the Pt layer 14 iscompleted by performing the lift-off, to remove resist(s) (and the metalcovering that resist) from unwanted regions (step 36 e). A conventionalsolvent, such as N-methyl-pyrrolidone, followed by a water rinse, may beused. Again, combinations of techniques well known in the semiconductorart, though not as preferred, may be used. If no pre-patterning wasdone, then at this step, the blanket platinum would be masked andetched, again, using techniques well known in the art.

Platinum may alternatively be deposited by evaporation, such as e-beamevaporation, also blanketly deposited.

The remaining Pt bottom conductor areas 14 are cleaned, which again issystem-dependent (step 36 f). If there is resist remaining from aprevious step, this step serves to remove any residual material. Theremoval of such residual material could be as restrained as thecleaning/tailoring step described immediately below. Alternatively,depending on the quality and quantity of residual material, the removalstep could be much more aggressive, using various combinations of plasmaetching, wet or dry etching, etc.

In the preferred embodiment, step 36 f is omitted, and an O₂ plasma isused to clean, as well as rearrange and smooth the surface of theremaining Pt layer 14 (step 36 g). An example of such O₂cleaning/tailoring is performed in an RIE System 1700; the conditionswere 80 sccm O₂, 100 mTorr, 100 Watts, for 5 minutes, operating underforward power control with a HIVAC base pressure of 2.0×10⁻⁵ Torr. Itappears that the surface is physically distinct, based on Atomic ForceMicroscopy images. It appears that the oxygen plasma is sufficient tocause some physical bombardment of the surface. At lower powers withhigher pressures, no rearrangement of the surface is observed.

Essentially, at relatively low pressure and high power (not too much gasin the chamber, physical bombardment), there is a sputtering componentthat increases with the mass of the species. On the other hand, atrelatively high pressure, low power (lots of gas; less acceleration),then mostly a chemical reaction occurs. Under the conditions of moderatepressure and power is where the desired rearrangement is obtained. Aswith the foregoing processes, this step is machine-dependent, and theoperating parameters will vary from one machine to another. However, thedetermination of such operating parameters for a specific machine is notconsidered to be undue, based on the teachings herein.

The tailoring step is performed in an oxygen plasma to rearrange theplatinum layer and to smooth the top surface of the platinum layer. Thisstep alters the hydrophilicity of the Pt layer to render it morehydrophilic and also provides a barrier layer (due to the presence ofthe PtO₂ on the surface). This is important, since the Pt surface isvery hydrophilic when the oxide is present and seems to be the key toobtaining a desirable uniform Langmuir-Blodgett film.

An oxygen plasma, as described in the previous paragraph, provides ahydrophilic Pt surface. Use of an oxygen plasma and a subsequent argonplasma may alternatively be used; this combination provides a lesshydrophilic, more hydrophobic Pt surface. Yet alternatively, an argonplasma alone may be used, which also provides a hydrophobic surface.Finally, a sequence of oxygen, then hydrogen plasmas may be used, toprovide a smooth surface with reduced oxygen, which is passivated.

The foregoing Pt deposition procedure yields a surface roughness that isless than 8 Å RMS, and can be as small as 4 Å RMS which is about as goodas the substrate coating 12 a. It also yields at this point anoxygenated surface and a hydrophilic surface.

Without subscribing to any particular theory, it appears that the reasonwhy a smooth platinum surface is obtained is based on the following: (1)prior to the platinum deposition, the process starts with smoothsurface, with smooth oxide thereon (or cleaved insulator, such as mica);(2) no sticking layer is used for adhesion of the Pt layer (stickinglayers, such as Ti, Cr, Ta, increase the surface roughness); and (3)subsequent to Pt deposition, the O₂ plasma removes any remainingpolymer, rearranges and smoothes the surface, without pitting it,thereby tailoring the Pt top surface. It will be appreciated that the O₂plasma also rearranges and smoothes even when no polymer (the resist)contact is initiated.

The oxygenated layer may be removed in an argon plasma in the same RIEmachine, either immediately following or at a later time. The conditionsof 40 mTorr, Ar (80 sccm), and 15 W forward power remove the oxygenatedlayer, maintain the smoothness of the rearranged surface, and produce asurface which wets identically to “as-deposited” platinum, with onlytrace amounts of oxide present.

EXAMPLES

Experimental Procedure

Both the blanket and photolithographically-modified Pt films weresputter deposited on Si wafers with a 100 nm silicon dioxide layer. Thetypical Pt thickness was 100 nm. The plasma treatment was performed in aRIE® model 1700 system. Freshly deposited Pt films and films exposed tovarious plasma treatments were analyzed with contact angle andellipsometry measurements within 10 minutes of preparation and by XPSand Auger with controls.

For contact angle measurements a droplet of 2 μL 18 MΩ·cm water wasinjected onto the sample surface from a syringe. An image of the staticwater droplet was recorded with a digital camera and analyzed to yield asessile contact angle, averaging at least three readings.

Ellipsometric measurements were performed using a laser with awavelength of 532 nm and an incident angle of 58 degrees. A simple modelwas used to derive the optical constants, n and k. The platinum wasapproximated by an infinite thickness. The reported values represent anaverage of three readings from different locations.

The surface morphology of the Pt films was monitored with a commercialatomic force microscope operated under ambient conditions in tappingmode. The surface roughness is calculated over a 1 μm² area.

XPS spectra were acquired on either a Surface Science Instrumentsspectrometer or a PHI Quantum 2000 spectrometer with monochromated Al Kα1486.6 eV X-ray source. Take-off angles in the two instruments were setat 35° and 45°, respectively. All the photoemission peak positions werecorrected to opportunistic C1s at 284.8 eV binding energy.

Auger analysis was performed on a PHI 670 Scanning Auger Microprobe witha CMA analyzer, 20 KeV, 10 nA beam energy and 45 degree tilt.

Results and Discussions

A. Optical Constants

Previous ellipsometric study has shown that the optical constants of Ptthin films were strongly dependent on the film deposition conditions. Inthis study, the optical constants, refractive index (n) and extinctioncoefficient (k), of films with different plasma treatments were derivedfrom single-wavelength ellipsometry with a single-layer model. The filmswith different plasma treatments fell into two classes based on theiroptical constants measured at 532 nm: a larger value class with n ˜2.5and k ˜4.2 and a smaller value class with n ˜1.8 and k ˜3.4. The filmstreated with argon plasma and those treated with argon after oxygenbehaved similarly to the as-deposited film. They all exhibited largeroptical constants. In contrast, measurements of the platinum filmsexposed only to oxygen plasma resulted in optical parameters belongingto the smaller values class. Films intentionally introduced tophotochemicals before plasma treatment showed no variation from theabove.

Although there was only a slight decrease of the n and k values overseveral hours, contact angle measurements exhibited a larger change.Ellipsometry appears not to be sensitive to the changes that do occur.

B. Contact Angle Measurement

Water contact angle is a direct measure of surface hydrophilicity.Sessile water contact angles of the Pt thin films were recorded inparallel with the optical constants. Under ambient conditions, contactangles increased markedly within in the first three hours, changingslowly thereafter. As a catalytic material, a variety of chemicalspecies can adsorb onto platinum surfaces. As the surface adsorbs CO,hydrocarbons, and other organic compounds, the surface free energydecreases and a higher water contact angle is observed. Contact anglestudies by other investigators also have documented a hydrophilic naturemigrating toward hydrophobic within minutes of exposure to thelaboratory atmosphere. Hydrophobic is defined as a contact angle greaterthan 30 degrees.

The platinum films could also be divided into two classes, based uponthe time dependence of the water contact angle. The samples in thehigher contact angle group consisted of: the fresh as-deposited film andfilms treated with an argon plasma. The samples exhibiting values in thelower contact angle group were the films treated with an oxygen plasma(and no subsequent argon plasma). This is consistent with theellipsometric measurements.

Both measurements reveal that an oxygen plasma treatment changes someplatinum thin film properties, while an argon plasma treatment canrestore some properties of freshly deposited Pt films. The oxygen plasmatreated surfaces are initially more hydrophilic than the freshlydeposited or argon plasma treated surfaces, but the rate of increase ofthe contact angle is similar for both classes. In order to understandwhy and how the oxygen plasma treatment can change surface properties sodramatically, x-ray photoelectron spectroscopy was utilized to examinethe surface chemical composition of the platinum thin films.

C. X-Ray Photoelectron Spectroscopy (XPS) and Auger ElectronSpectroscopy (Auger)

The survey and Pt 4f region spectra of four platinum thin films werescanned. The four films were (1) a fresh as-deposited thin film, (2) afilm treated with argon plasma (5 min. at 100 W and 100 mTorr “AR1”)alone, (3) a film treated with only oxygen plasma (5 min. at 100 W and100 mTorr; “OX1”), and (4) a film treated with oxygen plasma (5 min. at100 W and 100 mTorr) followed by argon (5 min. at 100 W and 100 mTorr)plasma. Only Pt, C, and O were observed on all samples. The presence ofcarbon and oxygen was unavoidable because of surface adsorption ofhydrocarbons and species with C—O functionalities. The peak position andintensity of C, O, and Pt were almost identical on the freshas-deposited thin film, the film treated with argon plasma, and the filmtreated with oxygen plasma plus argon plasma. However, a significantincrease of the O 1s peak intensity at 532 eV was observed in the filmtreated with oxygen plasma alone. In addition, a new set of Pt 4f peaksappeared on this sample at higher binding energy. The new peaks, Pt4f_(7/2) at 74.7 eV and Pt 4f_(5/2) at 78.0 eV, are conclusive evidenceof platinum oxide formation. This result is also consistent with the XPSresult for a previously reported PtO₂ thin film prepared by reactivesputtering in the presence of oxygen gas.

Combining all the pieces of information derived from optical constantmeasurements, contact angle measurement, XPS, and Auger studies, it isclear that the oxygen plasma treatment forms an oxide layer on the Ptthin film surface and changes the surface properties dramatically. Inorder to understand the relationship between oxide generation and theoxygen plasma condition, high-resolution spectra of platinum thin filmstreated with a somewhat aggressive oxygen plasma treatment (5 min. at100 W and 100 mTorr), OX1, and with a less aggressive plasma (2 min at50 W and 50 mTorr), OX2, were studied. The relative atomicconcentrations of all the fitted components are listed in Table 1, afterthe absolute peak areas were corrected with the sensitivity factor ofeach element. TABLE 1 The relative atomic concentration (%) of fittedpeaks at different chemical states. Pt 4f peaks O 1s peaks 2 + 2′ 1 21 + 1′ (PtO or 3 + 3′ (metal (C—O re- C 1s Samples* (Pt⁰) Pt(OH)₂)(PtO₂) oxide) lated) peaks OX1 6.4 4.5 17.1 32.5 19.1 20.3 OX2 7.9 5.116.7 30.7 20.2 19.9 OX1 + AR2 53.9 1.0 0.1 2.7 3.6 38.6 OX2 + AR2 55.10.8 0.0 3.1 2.2 38.8OX1 = O₂ plasma: 5 min. 100 W 100 mTorr; OX2 = O₂ plasma: 2 min. 50 W 50mTorr; AR2 = Ar plasma: 1 min. 15 W 40 mTorr.

The majority of the Pt, 56% to 61%, within the XPS sampling depth(usually less than 50 Å) of films treated with oxygen plasma was in thePtO₂ chemical state as denoted 3 and 3′. The O to Pt atomic ratio isnearly 2:1, provided that the Pt⁰ (denoted as 1 and 1′) was excluded inthese samples. A small portion of Pt, 16% to 17%, was assignedtentatively as PtO or Pt(OH)₂ chemical state as denoted 2 and 2′. Themore aggressive oxygen plasma produces only slightly more oxide than theless aggressive oxygen plasma, based on the ratio of Pt in oxidechemical states vs. Pt in the metallic state.

Estimation of thickness of platinum oxide from high-resolution XPSspectra was performed using the simple substrate-overlayer model and thethickness of oxide in the Pt film treated with the aggressive and lessaggressive oxygen plasmas was calculated to be 2.4 nm and 2.7 nm,respectively. Auger data, which follows, differs with respect to thisthickness.

XPS shows about 98% of Pt exists in the metallic chemical state (Pt⁰)after a further treatment with the AR2 argon plasma. The stated argonplasma condition is the minimal possible power and flow to generate astable plasma in the RIE instrument. Any platinum oxides were present inquantities below the XPS detection limit. The oxygen atomicconcentration dropped to less than 6% among the elements detected onthese samples and could be mainly attributed to the surface adsorbedspecies with C—O functional groups. A high percentage of C was alsodetected in these metallic platinum film surfaces from various adsorbedspecies.

The Auger Electron Spectroscopy results showed similar elements butdiffered with respect to oxide thickness. The elements detected on thesurface of each of the samples were primarily platinum plus carbon andoxygen. By elemental analysis of the etch products, seeking the point atwhich oxygen from the sample became undetectable during etching, it wasconcluded that the oxide (PtO, PtO₂, Pt(OH)₂) was less than 5 Å inthickness (for a sample treated with OX1), actual depth, full width,half maximum (FWHM). The oxygen content of as-deposited and OX1+AR2treated samples was minimal and their oxide thicknesses were less than 2Å.

The ion-gun etch rate was experimentally determined to be 5.2 Å/min(actual depth in Pt(O) by AFM measurement) The calculated conversionfactor between the Pt(oxide) etch rate and SiO₂ calibration material wasconsistent with that for other heavy metals. Survey scans of the sampleswere presented as plots of the first derivative of the number ofelectrons detected as a function of energy. Depth profiles were obtainedby alternating an acquisition cycle with a sputter cycle. During theacquisition cycle selected elemental peak intensities were collected.The sputter cycle removed material from the surface of the sample usinga 2 keV Ar⁺ source rastered over a 5 mm×5 mm area. In order to eliminatecrater wall effects, the data was acquired from a much smaller region inthe center of the sputtered area.

For a sample subjected to OX1, slight shifts in the platinum peakposition due to chemical state allowed the Pt (oxide) and Pt (metal)components of the metal to be separated using a linear least squares(LLS) curve fitting routine. No correction to the relative sensitivityfactor was made for the Pt (oxide) trace for stoichiometry and thereforeerror may be present in the atomic compositions reported.

The PtO₂ peaks dominate the OX1 spectrum where ˜61% of the Pt is presentas PtO₂. The remaining Pt is present in two or three different statesand in the initial XPS data these states were separated into Pt⁰ (metal)and PtO/Pt(OH)₂. Due to the strong peaks of PtO₂ and PtO, the PtO andPt(OH)₂ chemical states could not be accurately separated.

Using the OX1+AR2 treated sample as a reference for spectral subtractionand assuming that this sample is representative of the surface aftercleaning and after exposure to air, the reference spectrum of the samplewith treatment OX1+AR2 is seen as primarily Pt⁰ with trace amounts ofPtO/Pt(OH)₂. Scaling and subtracting the spectrum of the sample treatedwith OX1+AR2 from that treated with OX1 alone produces the chemicaldifference between the two samples, i.e., the effect of the oxygenplasma. In this subtracted spectrum, the primary peaks are associatedwith the presence of PtO₂ but minor states are also present.Curve-fitting the spectrum reveals PtO₂ and two additional chemicalstates that correlate to PtO and Pt(OH)₂. The data shows anapproximately 2 eV difference between these two chemical states, whichis corroborated by available literature. The narrowness of the fittedpeaks cause some ambiguity as to the precise ratios of these twochemical states, but both are present in the sample treated with OX1.

The ratios of PtO2:PtO:Pt(OH)₂ were found to be:

-   -   PtO₂: 87.4%    -   PtO: ˜5.1%    -   Pt(OH)₂: ˜7.5%

In conclusion, the spectral subtraction shows more clearly thedifference between samples treated with OX1 alone and OX1+AR2. Thesedifferences include the presence of three additional chemical states forplatinum: PtO₂ (predominantly) and lesser amounts of both PtO andPt(OH)₂.

D. Atomic Force Microscopy (AFM)

Plasma treatment of the platinum thin films also altered the morphology.Investigation was carried out to achieve surfaces with as smooth aspossible morphology. The surface roughness was monitored by AFM, and thedata is listed in Table 2, along with other surface properties. Thesputtering deposition condition used in this laboratory producesplatinum thin films with RMS roughness of 5.4 Å over an area of 1 μm².TABLE 2 The surface properties of platinum thin film treated withdifferent plasma conditions. Water Contact RMS angle roughness Processcondition * (degrees) n k in 1 μm² (Å) Fresh as-deposited Pt 32 2.534.26 5.4 5 min O₂ plasma (OX1) alone w 1.85 3.35 3.4 5 min Ar plasmaalone 30 2.47 4.18 8.1 OX1 + 5 min Ar plasma 30 2.50 4.21 5.7 OX1 + 3min Ar plasma 25 2.51 4.23 6.0 OX1 + 1 min Ar plasma 25 2.45 4.15 5.6OX1 + 1 min Ar plasma (50 W, 31 2.48 4.18 4.8 50 mTorr) OX1 + 1 min Arplasma (25 W, 32 2.47 4.18 4.4 50 mTorr) OX1 + 1 min Ar plasma (15 W, 272.40 4.07 3.8 40 mTorr) (AR2) OX1 + 1 min Ar plasma (20 W, w 1.90 3.413.1 25 mTorr, no plasma is generated)

O₂ or Ar plasma: 100 W 100 mTorr, unless otherwise specified. w: waterreadily wetted the surface producing a contact angle of generally lessthan 10 degrees, so it was difficult to obtain an accurate reading.

Argon plasma exposure, particularly, “high” power plasma, will roughenthe platinum surface. An 8.1 Å RMS roughness was observed for thesurface treated with argon plasma for 5 min. at 100 W and 100 mTorr.Heavy argon atoms under a high power plasma condition can bombard the Ptthin film and roughen the surface. Oxygen plasma exposure did notroughen the surface, but rather smoothed it, as suggested by a 3.4 Åroughness over an area of 1 μm² recorded for the surface treated oxygenplasma for 5 min. at 100 W and 100 mTorr.

A series of lower power/shorter duration argon plasmas was evaluated forits ability to minimize the effect of roughening. By using a minimalargon plasma, 1 min. 15 W at 40 mTorr, little roughening (3.8 Å RMSroughness in 1 μm²) of the platinum thin film surface occurred, yet theoxide was removed and surface properties dramatically changed.

Conclusion

The properties of platinum thin films are strongly affected by theplasma treatment conditions. Argon-treated Pt thin films behavedsimilarly to as-deposited untreated films with respect to water contactangle and ellipsometrically measured optical properties. Oxygen plasmatreatment resulted in marked change of the surface chemical properties.XPS and Auger studies confirmed the formation of platinum oxides, PtO₂,PtO and Pt(OH) after the film was treated with oxygen, even under modestplasma conditions. The change in the surface properties was attributedto the formation of such an oxide layer on the film surface. Furthertreatment with argon plasma diminished the oxide layer; however,aggressive argon plasmas roughened the surface. In order to minimize thesurface roughness, a minimal argon plasma recipe subsequent to oxygenplasma treatment was developed to produce clean, metallic Pt thin filmswith a roughness of less than 4 Å within a 1 μm² area.

Initial experiments indicate that hydrogen plasma will also remove theoxide and may offer some passivation advantages.

INDUSTRIAL APPLICABILITY

The method of fabricating a platinum layer having a relatively smoothsurface and tailored mechanical, physical and chemical properties in amolecular electronic device is expected to find use in nanoscalecomputing and memory circuits.

1-51. (canceled)
 52. A conductive layer having a surface roughness ofless than 8 Å RMS.
 53. A conductive layer formed on a substrate andhaving a surface roughness essentially the same as that of saidsubstrate.
 54. The conductive layer of claim 52 having a thickness ofabout 500 to 5,000 Å.
 55. The conductive layer of claim 54 having athickness of about 1,000 Å.
 56. The conductive layer of claim 52,wherein said conductive layer is supported on a substrate.
 57. Theconductive layer of claim 56 wherein said substrate comprises a materialselected from the group consisting of silicon and insulating materialsother than silicon.
 58. The conductive layer of claim 57 wherein saidsubstrate comprises silicon, and an oxide or nitride layer thereon, saidconductive layer on said oxide or nitride layer.
 59. The conductivelayer of claim 57 wherein said substrate comprises mica, said conductivelayer on said mica substrate.
 60. The conductive layer of claim 52wherein said conductive layer is either hydrophobic or hydrophilic. 61.The conductive layer of claim 52 wherein said surface roughness is lessthan 4 Å RMS.
 62. The conductive layer of claim 53 wherein said surfaceroughness is of less than 8 Å RMS.
 63. The conductive layer of claim 62wherein said surface roughness is less than 4 Å RMS.
 64. The conductivelayer of claim 53 having a thickness of about 500 to 5,000 Å.
 65. Theconductive layer of claim 64 having a thickness of about 1,000 Å. 66.The conductive layer of claim 62, wherein said conductive layer issupported on a substrate.
 67. The conductive layer of claim 66 whereinsaid substrate comprises a material selected from the group consistingof silicon and insulating materials other than silicon.
 68. Theconductive layer of claim 67 wherein said substrate comprises silicon,and an oxide or nitride layer thereon, said conductive layer on saidoxide or nitride layer.
 69. The conductive layer of claim 67 whereinsaid substrate comprises mica, said conductive layer on said micasubstrate.
 70. The conductive layer of claim 53 wherein said conductivelayer is either hydrophobic or hydrophilic.